Life on land prior to the origin of land plants: fossil evidence and palaeoenvironmental significance

大约5.4亿年前，海洋中的多细胞生命发生了戏剧性的多样化。所谓的寒武纪大爆发见证了海洋中丰富、多样和不同的多细胞动物、真菌和藻类的出现和多样化。然而，令人惊讶的是，尽管海洋中充满了复杂的生命，但陆地表面被认为基本上是贫瘠的。直到大约7500万年后的中奥陶世，才出现了第一个陆地植物的证据。为什么在复杂的多细胞生物(植物)入侵陆地之前有如此广泛的滞后？在中奥陶世之前，陆地上生活着什么？该项目的目的是研究从古代大陆沉积物中新发现的化石，这些化石表明，在陆地植物进化之前，陆地表面存在简单的藻类植被。长期以来，人们一直怀疑，在陆地植物起源之前，大陆上可能存在一种简单的生物群，通常被轻蔑地称为“藻类浮渣”。据推测，这种生物群由简单的光合作用有机体组成，如细菌和藻类，也可能包括分解者如真菌。这种生物群可能居住在各种环境中，并利用各种生活方式，包括(I)在淡水水体(湖泊、河流等)中自由生活；(Ii)在定期被淹没的地区(如河口泥滩)上作为藻席；(Iii)在原始土壤内和表面上作为生物土壤结壳；(Iv)在岩石中(作为内石层)。遗憾的是，这主要是猜测。除了关于简单化石和微弱的地球化学信号的极其罕见但诱人的报道外，几乎没有确凿的证据证明有这样的地球生命。新发现的化石材料由简单的球形囊状结构(球形)、管状和细丝以及各种其他有机微体化石组成。微体化石代表了栖息在水生环境中的生物，也代表了居住在周围陆地表面(土壤结壳等)的生物。它们被冲进了水环境中。它们清楚地表明了早期陆地生命的存在。但挑战是理解它们代表的是什么类型的有机体？该计划是将它们与被认为代表古代陆地生态系统的生物材料进行比较。今天，除了最冷的极地地区、高山和最干燥的沙漠外，陆地植物栖息在地球的所有表面。这些不适宜居住的环境中居住着藻类垫子、生物土壤结壳和石器时代的群落，据信这些群落代表着陆地植物起源之前的残余群落。这些社区的生命将从北美和澳大利亚的炎热沙漠以及冰岛的寒冷沙漠中收集。它将被实验性地转化为化石材料，有望在化石记录中幸存下来。然后将活体材料和化石材料进行比较，以确定这些化石到底代表了什么。对古陆相沉积物中化石的分析将极大地帮助我们理解陆前植物生物的本质。显然，一个“藻类浮渣”在陆地表面栖息了很长一段时间。这具有巨大的进化和古生态意义，除了对了解陆地植物起源之前的环境和全球变化的性质具有巨大的影响外，目前的陆地植被约占地球生物量的90%，并对环境产生重大影响(通过固碳、土壤形成和风化等影响气候、大气组成和沉积模式)。这种经常被忽视的陆生植物生物群到底有多重要，它通过早期陆基生物地球化学循环对环境产生了多大影响？

Approximately 540 million years ago there was a dramatic diversification of multicellular life in the oceans. The so-called 'Cambrian explosion' witnessed the appearance and diversification in the oceans of abundant, diverse and disparate multicellular animals, fungi and algae. Rather surprisingly, however, whilst the oceans teemed with complex life, the land surface is considered to have been essentially barren. It wasn't until the Mid Ordovician, some 75 million years later, that the first evidence for land plants appears. Why was there such an extensive lag before the invasion of the land by complex multicellular organisms (plants) and what, if anything, lived on the land prior to the Mid Ordovician? The aim of this project is to study newly discovered fossils from ancient continental deposits that suggest that the land surface harboured a simple vegetation of algae before the land plants evolved. It has long been suspected that prior to the origin of land plants the continents may have harboured a simple biota often referred to, rather disparagingly, as an 'algal scum'. It has been postulated that this biota consisted of simple photosynthesizing organism such as bacteria and algae, and possibly also decomposers such as fungi. This biota probably inhabited a variety of environments and exploiting a variety of lifestyles including (i) free living in bodies of freshwater (lakes, rivers etc.); (ii) as algal mats on periodically inundated areas (such as estuary mud flats); (iii) within and on the surface of rudimentary soils as biological soil crusts; (iv) within rocks (as endoliths). Sadly this is mostly speculation. There is very little firm evidence for such terrestrial life, barring extremely rare but tantalizing reports of simple fossils and faint geochemical signals. The newly discovered fossil material consists of simple, spherical, sac-like structures (sphaeromorphs), tubes and filaments, and various other organic microfossils. The microfossils represent organisms that inhabited aquatic environments, but also those inhabiting the surrounding land surface (soil crusts etc.) that were washed into the aquatic environments. They clearly indicate the presence of early terrestrial life. But the challenge is understanding what types of organism they represent? The plan is to compare them with living material believed to represent counterparts of the ancient terrestrial ecosystems. Today land plants inhabit all of the Earth's surface except for the coldest polar regions and high peaks and the driest deserts. These inhospitable environments are inhabited by algal mats, biological soil crusts and endolithic communities believed to represent relict communities from before the origin of the land plants. Life from these communities will be collected, from the hot deserts of North America and Australia and the cold deserts of Iceland. It will be experimentally turned into fossil material that might be expected to survive in the fossil record. Then the living and fossil material will be compared to determine what the fossils actually represent. Analysis of fossils recovered from ancient terrestrial deposits will greatly illuminate our understanding of the nature of pre-land plant terrestrial biotas. Clearly an 'algal scum' inhabited the land surface for a very long time. This has immense evolutionary and palaeoecological significance, in addition to enormous implications for understanding the nature of the environment and global change prior to the origin of land plants The current terrestrial vegetation constitutes some 90% of the living biomass of the planet and has a major impact on the environment (affecting climate, atmospheric composition and pattern of sedimentation through carbon sequestration, soil formation, weathering and so on). Just how significant was the oft neglected pre-land plant terrestrial biota and how much did it influence the environment through early terrestrial-based biogeochemical cycles?